The invention relates to a process for recycling fuel cell components, in particular to obtain precious metals and/or fluorine-containing constituents from fuel cell components, for example from PEM fuel cell stacks, DMFC fuel cells, catalyst-coated membranes (CCMs), electrodes or membrane-electrode units (MEUs), in a more concentrated form. The fluorine-containing constituents are separated from the residues containing precious metals by treatment in a supercritical medium. The precious metals are recovered from the residues without harmful fluorine or hydrogen fluoride emissions in conventional processes.
The process is employed in the recovery of precious metals and/or fluorine-containing ionomer materials from fuel cells, electrolysis cells, batteries, sensors and other electrochemical devices.
The energy conversion process in a fuel cell is largely free of pollutants and has a particularly high efficiency. For this reason, fuel cells are becoming increasingly important for alternative drive concepts, domestic energy supply plants and also portable applications.
PEM fuel cells are made up of a stack of many fuel cell units. These are electrically connected in series to increase the operating voltage. The key component of a PEM fuel cell is the Membrane-Electrode Unit (MEU). The MEU comprises the proton-conducting membrane (polymer electrolyte membrane or ionomer membrane), the two gas diffusion layers (GDLs or “backings”) on the sides of the membrane and the electrode layers located between membrane and gas diffusion layers. One of the electrode layers is configured as anode for the oxidation of hydrogen and the second electrode layer is configured as cathode for the reduction of oxygen.
Depending on the specification and field of use, the catalyst components in fuel cell stacks contain considerable amounts of precious metals such as platinum, ruthenium, palladium and others. For example, a 50 kW PEM stack as is currently used for mobile applications in passenger cars contains from about 50 to 100 grams of platinum (i.e. about 1-2 g of platinum/kW). The widespread introduction of fuel cell technology for automobiles involving large numbers of fuel cells would mean the provision of considerable quantities of platinum, at least for the first generation of vehicles. In addition, there then has to be a recovery process available for the precious metals bound in the fuel cell stack so that a closed loop for precious metals and thus the supply of precious metals is ensured.
Apart from the precious metal catalysts, the membrane materials have the highest material costs. The polymer electrolyte membrane comprises proton-conducting polymer materials, hereinafter also referred to as ionomers for short. Preference is given to using a tetrafluoroethylene-fluorovinyl ether copolymer bearing sulphonic groups. This material is produced in expensive and complicated processes and is marketed, for example, under the trade name Nafion® by DuPont. For use in fuel cells, these membranes generally need to have a thickness of from 20 to 200 μm, so that considerable amounts of membrane materials are present in fuel cell stacks.
The electrode layers for anode and cathode contain not only the proton-conducting fluorine-containing polymer (Nafion®) but also electrocatalysts which catalyse the respective reaction (oxidation of hydrogen or reduction of oxygen). As catalytically active components, preference is given to using the metals of the platinum group of the Periodic Table of the Elements (Pt, Pd, Ag, Au, Ru, Rh, Os, Ir). In the majority of cases, use is made of supported catalysts in which the catalytically active platinum group metals have been applied in finely divided form to the surface of a conductive support material, for example carbon black.
The gas diffusion layers (GDLs) generally comprise carbon fibre paper or woven carbon fibre fabrics which are usually hydrophobicized with fluorine-containing polymers (PTFE, polytetrafluoroethylene, etc.). They make it possible for the reaction gases to gain ready access to the reaction layers and allow the cell current and the water formed to be conducted away readily.
In the construction of fuel cell stacks, GDLs and MEUs are stacked on top of one another using bipolar plates. The sequence is generally: end plate—GDL (anode)—CCM—GDL (cathode)—bipolar plate—GDL (anode)—CCM—GDL (cathode)—bipolar plate (etc.)—end plate. Depending on the required power range, up to 100 MEUs are stacked on top of one another in a stack.
The bipolar plates generally comprise of conductive carbon, preferably graphite. They contain channels in a particular pattern through which gas supply (fuel gas to anode and air to cathode) is effected in the stack. During recovery of precious metals and membranes from the PEMFC stack, the bipolar plates can in principle be separated off during disassembly of the stack and be reused. However, processes in which the entire stack (including the bipolar plates) is passed to recovery are also possible.
Commercialization of PEM fuel cell technology requires not only industrial production processes for catalyst-coated membranes (CCMs), catalyst-coated gas diffusion layers (CCBs) and Membrane-Electrode Units (MEUs) but also industrial and rational processes for recovering the precious metals and the expensive ionomer membranes. Only the use of such processes will make fuel cell technology economically and ecologically feasible. The provision of appropriate recycling processes creates the preconditions for fuel cell assemblies being able to be introduced on the market in large numbers for mobile, stationary and portable applications.
There are only few examples of the recycling of precious metal-containing fuel cell components and the recycling of ionomer membranes in the literature.
Conventional combustion processes are known for obtaining precious metal concentrates from catalysts. Residues of catalysts having combustible carbon supports (for example Pd/activated carbon) are burnt in gas furnaces and the precious metal-containing ash is worked up. The precious metal concentration after ashing is normally sufficiently high for direct digestion using wet chemical methods (cf. C. Hagelueken, “Edelmetalleinsatz und—Recycling in der Katalysatortechnik”, Erzmetall 49, No. 2, pages 122-133 (DZA Verlag für Kultur und Wissenschaft, D-04600 Altenburg).
WO 01/83834 A1 discloses a process for recovering precious metals from organic precious metal-containing materials, in which organic impurities and residues are removed by means of supercritical water and oxygen in an oxidation process.
WO 81/00855 teaches a method for treatment of organic materials in supercritical water. The feed organic materials are restructured to form resulting organic materials including non-toxic materials from toxic starting materials and useful volatile organic liquids.
U.S. Pat. No. 5,133,843 proposes a process comprising dissolution of the precious metals in aqua regia for the work-up or recovery (“rejuvenation”) of an ionomer membrane coated with precious metals. The ionomer membrane can then be reused for fuel cells.
JP 11,288,732 describes a method of recovering components for fuel cells, in which the membrane-electrode units are treated with a solvent which dissolves the fluorine-containing ionomer or the membrane. The fluoropolymer is in this way separated off from metallic catalysts and other insoluble constituents. A disadvantage of the process is the use of organic solvents which present problems in respect of flammability, safety, environmental pollution and toxicity. The further work-up of the fluorine-containing catalyst components is not described.
The recovery of used perfluorinated sulphonic acid membranes is described by H-F. Xu, X. Wang et al. in Journal of Applied Electrochemistry (2002), 32 (12), pages 1337-1340. The Nafion® membrane is dissolved in DMSO at 170° C. and atmospheric pressure and is subsequently recovered in a “recast” process. Here too, the use of organic solvents is a disadvantage.
In the direct pyrolytic work-up of fluorine-containing fuel cell components and composite materials (for example PEM stacks, MEUs, GDLs and catalyst-coated ionomer membranes), hydrogen fluoride (HF) is formed from the organic polymers during combustion. This gas is present in the combustion gases, so that an additional purification device for its removal is necessary. Furthermore, owing to its toxicity and corrosive properties, hydrogen fluoride requires specific safety measures, for example pipes, filters and scrubbers made of stainless steel. For these reasons, the direct pyrolytical concentration of precious metals from fluorine-containing fuel cell components has hitherto been associated with great technical problems.
Furthermore, the fluorine constituents have to be removed from the precious metal-containing slag or mixture, since they would interfere in the later work-up process or in the separation of the precious metals and lead to a reduction in yield. For this reason too, the fluorine-containing constituents have to be separated from the precious metal-containing constituents.